Recently, the reconfigurable intelligent surface (RIS) has attracted far-ranging attention due to its low profit, low energy consumption, and ability to overcome the non-LoS (NLoS) transmission [1]}. Typically, the RIS contains many artificially configurable elements, each of which is manipulated to induce changes in the amplitude and phase shift of incident signals. Therefore, the RIS becomes the preferable technology for the future sixth-generation (6G) mobile communications. It can solve the pain points of 5G, such as high energy consumption and coverage voids. By optimizing the phase shift of all elements of the RIS, the signals from different transmission paths can be precisely aligned at the desired receiver to increase signal energy [2]}.
